In many video systems, various types of information, either from different sources or from the same source, are available for display on a display monitor. Such types of information may for example be graphics, alphanumerics, cursors or other control indications on the screen, or light or dark background areas for one of the above. An example of an application where such various types of information arise is where graphic information is being displayed and it is desired to provide an alphanumeric "window" over the qraphic display. In this application, a blanking of the window area might occur to provide a light background on which the alphanumerics are easily viewed, the alphanumerics possibly being obscured by portions of the graphics if both had the same or substantially similar gray scale values. Another example might be in industrial design or architectural applications where different sections are stored of a product or structure with it being possible to select material from different layers for simultaneous display.
In applications such as those described above, it is necessary to establish a priority among the various types of the information and to determine the highest priority type of information which is to be displayed at a given pixel position of the display during each display refresh cycle. One way in which this prioritization may be accomplished is to provide a memory plane bitmap for each type or level of information which is adapted to be displayed. When, either in response to a user input, to a processor output, or to some combination thereof, an indication is received that information of a particular type or level is to be displayed in a particular pixel position or area of the display, the bitmap plane for that level has bits written into it for the appropriate pixel positions. A mechanism may then be provided for determining the memory plane or bitmap having the highest priority (or at the highest level) which has a bit stored in it for each pixel position of the display an this determination is utilized to control the type of information which is displayed at each such pixel position.
Heretofore, performing the prioritization function indicated above has been relatively time consuming for a number of reasons. First, the video RAMs which are frequently utilized for performing such function are adapted to store and to read out only four bits at a time. Thus, for high resolution displays which may, for example, have 512.times.512 pixels, 1024.times.1024 pixels, or more, a very large number of read-out cycles from each of the priority bitmap memories is required in order to obtain a single bitmap and a large number of write cycles may be required to store a bitmap. Secondly, buffering and masking may be required in order to assure that comparable information from each of the bitmap planes is being simultaneously accessed so that meaningful comparisons can be made in the prioritizing circuitry.
An improved method and apparatus for performing the prioritization function in a video display system is therefore required which system permits information concerning a given information level or type to be stored as a bitmap as quickly as possible and which permits information for the various bitmaps which are to be compared to be simultaneously read out and utilized for the priority determination.